1. Field of the Invention
The present invention generally relates to photoelectric devices, and more specifically to a photovoltaic cell having a structurally supporting back electrode open conductive support or grid structure.
2. Description of the Related Art
A conventional photovoltaic or solar cell includes a photoresponsive sheet or layer of material which generates a photovoltaic effect in response to light incident on a front surface thereof. The photoresponsive layer may be formed of a single crystalline, semiconductive material such as silicon, and have differently doped strata which define a junction therebetween. Alternatively, the photoresponsive layer may include strata of dissimilar materials such as cadmium sulfide/copper sulfide, or aluminum gallium arsenide (AlGaAs) and gallium arsenide (GaAs). Front and back electrodes are formed on the front and back sides of the photoresponsive layer, respectively, in ohmic connection therewith. The back electrode is generally a continuous metal layer or film, since it is not required to transmit light therethrough. The front electrode may be formed of a transparent conductive oxide (TCO) such as indium tin oxide (ITO), a thin grid pattern of a metal such as copper, aluminum, silver, titanium/palladium/silver, or a combination thereof. Light incident on the front of the photoresponsive layer causes liberation of electron-hole pairs therein due to the photovoltaic effect. The electrons and holes flow out of the photoresponsive layer to an external load through one of the front and back electrodes, the direction of flow depending on the relative doping polarities of the strata in the photoresponsive layer.
U.S. Pat. No. 4,273,950, issued June 16, 1981, entitled "SOLAR CELL AND FABRICATION THEREOF USING MICROWAVES", to S. Chitre, discloses an example of a solar cell having a silicon photoresponsive layer, a continuous back electrode, and a front electrode grid including metal conductors which are 2,500 microns wide and 2 microns thick. The grid structure covers approximately 8 to 10% of the area of the front surface of the photoresponsive layer. This is a compromise between the objectives of maximizing the junction area that is exposed to light (which requires minimum conductor area), while at the same time maximizing the current collecting capability of the grid structure.
Another example of a front electrode grid structure is disclosed in U.S. Pat. No. 4,348,546, issued Sept. 7, 1982, entitled "FRONT SURFACE METALLIZATION AND ENCAPSULATION OF SOLAR CELLS", to R. Little. This grid is in the form of a grid of wires of sufficiently high tensile strength to be self-supporting while being drawn from spools or the like into contact with one or more components of the solar cell before completion of the cell's fabrication.
The above referenced patents each disclose a solar cell having a continuous back metal electrode. U.S. Pat. No. 4,795,500, issued Jan. 3, 1989, entitled "PHOTOVOLTAIC DEVICE", to Y. Kishi, discloses a solar cell having a front, transparent ITO electrode, and a back metal electrode which has holes formed therethrough. The purpose of the holes is to allow a part of the light incident on the cell to be transmitted therethrough for lighting the inside of a room, automotive vehicle, or the like.
Conventional back electrodes, whether or not they are formed with holes for transmissibility, are made quite thin, on the order of several microns. This dimension is consistent with their function of collecting current generated by the photovoltaic effect in the photoresponsive layer.
Although gallium arsenide (GaAs), or GaAs on Ge cells, generate 1/3 more power than standard silicon cells, they have the disadvantages of higher weight and manufacturing cost. Current techniques allow for thinning of GaAs solar cells to approximately 125 microns using a Strasbaugh grinding machine or etching process. Thinner cells are not cost effective due to excessive attrition through breakage.
Thin solar cells are further prone to cracking. Propagation of a crack perpendicular to the direction of current flow in a circuit having a number of cells connected in series has the potential of reducing the output of the circuit to zero. A standard practice for preventing a crack from breaking electrical conductivity in a series cell string is to weld or solder a T-bar strap across the back metal electrode of each cell. However, this adds weight to the cell, and requires 2 to 4 additional applications of localized heating which increases the risk of cell breakage through thermal stress.